Trung Bộ Kinh (Majjhima Nikaya) 3:72
SUJATO‘Aniccānupassī assasissāmī’ti sikkhati, ‘aniccānupassī passasissāmī’ti sikkhati;
They practice like this: ‘I’ll breathe in observing impermanence.’ They practice like this: ‘I’ll breathe out observing impermanence.’